A battery pack having a large number of battery cells (or battery modules) connected in series is generally used. For example, hybrid vehicles, electric vehicles, and the like employ such a battery pack as a power supply for driving a motor.
Japanese Patent Laying-Open No. 2004-134287 (PTL 1) discloses a failure detection device for such a battery pack. This failure detection device detects whether a battery pack having a plurality of series connected battery cells or a battery pack having a plurality of series connected sets of cell parallel circuits each having a plurality of parallel connected battery cells is electrically overcharged or overdischarged, and the battery pack's internal resistance is estimated and in accordance therewith an overcharge threshold value and an overdischarge threshold value are modified. When a battery cell or a cell parallel circuit has voltage thereacross exceeding the overcharge threshold value, an overcharge detection signal is issued, whereas when a battery cell or a cell parallel circuit has voltage thereacross below the overdischarge threshold value, an overdischarge detection signal is issued.
The failure detection device allowing a battery packs internal resistance to be estimated and an overcharge threshold value and an overdischarge threshold value to be modified in accordance therewith is believed to ensure that whether the battery pack fails can be detected early (see PTL 1).